1. Field of the Invention
The present invention relates in general to derailleur type bicycle shifting systems, and more particularly to such a system wherein front and rear derailleur mechanisms are precisely controlled by respective rotatable handgrip shift actuators.
2. Description of the Prior Art
There has been a long-felt but previously unfulfilled need in the art for a bicycle derailleur shifting system which does not require that a hand, or least a thumb, be removed from the handlebar during shifting. Many derailleur shifting devices are actuated by levers mounted on the down tube of the frame, while some are mounted on the top tube and others on the handlebar. Such levers mounted on the down tube or the top tube all require that a hand be completely removed from the handlebar during shifting. Some derailleur shifting levers mounted on the handlebar can be actuated by taking a thumb off the handlebar and pushing the lever with the thumb, but this also diminishes control of the bicycle, and is awkward, so most riders simply take their hand off the handlebar to move the shift lever. For both safety and convenience, it is desirable to be able to shift derailleur mechanisms with both hands right on the handlebars. Despite a long-felt need for such a derailleur shifting system, applicant is not aware of any prior art derailleur shifting system where the shifting events can be accomplished with both hands on the handlebar.
Typical prior art derailleur shifting mechanisms which require removal of the hand, or at least the thumb, from the handlebar are disclosed in the following U.S. patents: Ross U.S. Pat. No. 4,055,093; Hedrich U.S. Pat. No. 4,194,408; Cirami U.S. Pat. No. 4,201,095; Bonnard 4,384,864; and Strong U.S. Pat. No. 4,548,092.
There has also been a long-felt but previously unfulfilled need in the art for a bicycle derailleur shifting system which is capable of "overshifting," yet which is not undesirably complicated and expensive. Overshifting is movement of the chain beyond the destination sprocket, and then back into alignment with the destination sprocket. It has long been known in the art that such overshifting is desirable during down-shifting events for earlier and smoother shifts. Most derailleur shifting systems do not have any built-in mechanism for accomplishing such overshifting, and require that the rider deliberately move the shifting lever beyond the location of the destination sprocket and then back to the destination sprocket. This requires two rider inputs, one being a determination of the desired extent of overshift, and the other being the time duration of the overshift. Satisfactory overshifting by this means requires considerable skill.
Applicant is aware of two prior art patents which disclose bicycle derailleur shifting apparatus having a built-in overshift feature. These are Yamasaki U.S. Pat. No. 4,267,744 and Bonnard U.S. Pat. No. 4,384,864. Both of these are very complicated mechanisms. Each of these devices has a built-in determination of the amount of overshift travel, yet neither of them determines the timing of the overshift. This is left up to the rider, who must first move a lever to the overshift position, and then move the lever back to the normal shift position.
Another problem with the Yamasaki and Bonnard overshift mechanisms is that they each provide the same amount of overshift travel for each one of the sprockets of a rear derailleur freewheel. The problem with this is that in most, if not all derailleur systems, the most advantageous extent of overshift travel varies for different freewheel sprockets. The "chain gap" or chain span between the derailleur guide pulley and a freewheel sprocket is considerably larger for the smaller, higher gear freewheel sprockets than for the larger, lower gear freewheel sprockets, and a relatively long chain gap generally requires a larger amount of overshift than a relatively short chain gap for optimum shifting. Another problem with the built-in overshift features in both Yamasaki and Bonnard is that an optimum amount of overshift for the other freewheel sprockets is generally too much for the #1, lowest gear sprocket closest to the wheel. An optimum amount of overshift travel for the other freewheel sprockets is likely to cause derailling from the #1 sprocket, which could seriously damage the bicycle. Thus, since the overshift amount is the same for all sprockets, it is inherent that neither of the Yamasaki or Bonnard overshift mechanisms produces sufficient overshift travel for optimum down-shifting.
Another long-noted problem in the art which is heretofore been unsolved is the provision of an accurate front derailleur system capable of handling not only "parallel riding" but also "cross-over riding." For example, with a 2-chain wheel front derailleur system, for parallel riding the larger chain wheel will service the smaller rear freewheel sprockets, and the smaller chain wheel will service the larger freewheel sprockets. With cross-over riding, the chain may be crossed over from the larger chain wheel to relatively large freewheel sprockets, or the chain may be crossed over from the smaller chain wheel to relatively small freewheel sprockets. Such crossed-over chain locations have a propensity for causing undesirable "chain rasp," and the prior art solution to this problem was simply to provide a front derailleur chain cage having a relatively wide gap between the cage plates. While this may reduce chain rasp, it causes the further problems of inaccuracy in shifting, and frequent chain derailling.
A further problem in the art, which relates primarily to rear bicycle derailleur shifting systems, is that there are numerous points of lost motion in both the derailleur mechanism and its actuating cable which cumulatively add up to a considerable amount of overall lost motion, as for example from about 0.040 to about 0.070 inch. Applicant has found that for accurate index shifting, substantially all of this cumulative lost motion must first be taken up at the shift actuator before the actual shift increment of travel between adjacent sprockets is applied during a down-shifting event. Applicant is not aware of any specific consideration of this problem in the prior art, and in particular of any specific compensation for such cumulative lost motion for each of the various type of derailleur and cable systems currently available.
It has long been recognized in the art that rotary handgrip devices can be useful for controlling vehicle mechanisms, particularly on motorcycles, but also on bicycles. Several of such devices are disclosed in Braumandl French patent No. 829,283. However applicant is not aware of any such device having previously been employed in cooperation with derailleur bicycle shifting apparatus, and such is not taught or suggested by Braumandl.